The present invention relates to circuits for shaping waveforms, and to integrated circuits which use such waveform-shaping techniques.
A very broad class of analog applications require some variable waveshaping capability. One important area is in communications interfaces. An important example, within this area, is the T1 specification, which is very commonly used for telephone line interfaces. The Bell System specification for T1 transmission lines requires that the waveforms of pulse edges have a certain shape at the Bell System interface. (The specified waveform shapes include a certain amount of pre-emphasis, in the form of overshoot on pulse transition edges. This helps to compensate for frequency-dependent attenuation later in transmission.) In a Short Loop interface (which is very widely used to interface customer equipment to the telephone network) the customer's T1 transceiver may be separated from the Bell system interface by as little as 0 feet, or as many as 655 feet, of twisted pair line.
The attenuation and shape of the data signals will therefore be strongly affected by the length of line used, and by the characteristics of the transformer used at the output. Therefore, T1 transceivers for customer use cannot simply generate a single waveform for T1 line drivers, but must be able to generate differing amounts of pre-emphasis, depending on the length of twisted pair used in any particular installation.
This variable pre-emphasis must be achieved within a waveform template which is reasonably complex. FIG. 5A shows the standard waveforms for the permissible envelope of T1 pulse shapes (and, overlaid on that, an example of how the pulse shapes used for a short distance connection would be modified for a 300 foot connection or for a 600 foot connection). (Similarly, FIG. 5B shows the standard waveforms for the permissible envelope of CEPT pulse shapes.)
The conventional way to implement this variable pre-emphasis has been to use an oscillator which runs at a multiple of the T1 frequency, and to use this higher-frequency signal to control the times at which different output voltages at different subperiods of the basic T1 clock are connected to the output. (The basic T1 clock period is one over 1.544 MHz, or about 648 nanoseconds.) However, this approach has inherent limitations on its time-domain resolution.
The present invention provides a greatly improved capability in waveform shaping. This is particularly advantageous in T1 interface applications, but is also applicable to a tremendous variety of other applications.
In the presently preferred embodiment, an integrated circuit with programmable delay lines is used to implement waveform shaping capability. By using programmable delay lines, an improved time-domain resolution is available which is much less than one clock delay.
One advantage of this approach is that the support circuitry is simple. A simple rising edge is all that is required to start the delay line chain, and generate the output waveform. Further, if 1 nsec precision is required, a 1 GHz oscillator is not. A further advantage is that "slow" technologies like CMOS can thus be used to build chips which can precisely synthesize high-speed and complex waveforms.
In the presently preferred embodiment, programming of the delay elements is performed only once, at the initial set-up of the system. However, of course, the innovative teachings set forth herein can also be applied to systems where it is desired to generate a customized waveform on the fly, or switch between ones of a pre-stored library of customized waveforms. In the presently preferred embodiment, the delay line alterations are performed by laser trimming of capacitors in a delay stage. However, other methods of delay programmation can be used instead.
By using the outputs of a programmable multi-stage delay line to drive selection gates which are connected to pass various outputs of a voltage-divider network, a desired waveform can be tailored very accurately.
It is most preferable, where a broad temperature range must be endured, to use delay stages which are as nearly equal as possible. (Otherwise, separate temperature compensation might be required for each stage.) However, where broad temperature range is not an absolutely paramount consideration, the unequal delays which are inherently available with these innovative teachings may be quite advantageous.
An innovative teaching set forth in the present application is a digital to analog converter (D/AC) which is metal-programmable to achieve a desired output waveform. This converter receives control lines from a multi-stage delay line. The delays between the various phases provided from the delay line can either be preset, or may be programmable on the fly. A resistor divider network (or other reference voltage-generating means, such as a capacitor array) is used to generate a set of voltage levels, and an array of transistors is connected so that each of the control lines selects one of the voltage levels for output.
Again, in the presently preferred embodiment, the different voltage values are initially maintained constant, but alternatively active devices could be used in analog mode, or to switch resistors in or out of the string, to change the tap voltages on fly.
In the presently preferred embodiment, a matrix of switching transistors provides unique advantages of late programmability. Each of the fractional output supply voltage lines is connected through a series of pass transistors, each of which is gated by one of the outputs from the multi-stage delay line taps. Metal options are available to short out each of the pass transistors.
Moreover, metal options can also be used to connect these gated output lines to multiple final output lines. Thus, these metal programmation options can be used to provide multiple output waveforms with a precise relative timing. In the presently preferred embodiment, this capability is used to provide fully complementary (mirror-image) waveforms. However, of course, this capability could also be used to implement more complex functionality.
The advantage of metal-level options is that late modification of the circuit's functionality is possible. This is well recognized in the semiconductor industry.
Several techniques are available to generate a desired waveform, from diode networks to digital-to-analog converters and oscillator dividers. The disadvantage of an oscillator divider to set the time "pickets" in a waveshaping system is the limited number of time points available: the total number of time points available per period is equal to the oscillator frequency f.sub.osc divided by the data rate (1.544 MHz in T1). Such a system is shown in FIG. 4. This limited time-domain resolution means that precise shaping of a high speed waveform is difficult.
The present invention permits the divider and oscillator to be replaced with precision delay elements that can be easily programmed.
In the T1 standard, the lowest time resolution which is recognized is 125 nanoseconds. Since the presently preferred embodiment provides time-domain resolution of approximately 1 nanosecond, the capabilities thus provided are far in excess of that required by the T1 standard.
The present invention is advantageous for many applications where a complex waveform needs to be reproduced repeatedly. For example, the present invention can be very advantageous for applications such as speech or music synthesis.
Many uses of a digital-to-analog converter are driven by an end requirement of shaping waveforms, and the instantaneous conversion of bits to voltages is merely a means to that end. Thus, the innovative circuits and architectures taught by the present invention provides an architecture which departs significantly from the normal ways of characterizing the performance of digital-to-analog converters, but which in many cases will be much more advantageous to users than a conventional digital-to-analog converter would be.
Note that a tapped delay chain (including adjustable delay elements) has been used to control sampling times for capturing high-speed analog signals. See U.S. Pat. No. 4,763,105 to Jenq (which is hereby incorporated by reference). However, this patent is primarily directed to high-speed analog-to-digital conversion (using low-speed sampling at successive times), and does not appear to suggest any relevance to digital-to-analog conversion or waveshaping.